In order to decrease a positive electrode potential during over-charge, a positive electrode capacity has been made excessive with respect to a negative electrode capacity, thus far. This is a strategy that takes advantage of the fact that by making the positive electrode capacity larger than the negative electrode capacity, it is possible suppress an increase in the potential of the positive electrode during over-charge and thereby suppress rapid heat generation.
In a cell in which the ratio p/n of the positive electrode capacity to the negative electrode capacity largely exceeds 1, the potential of a negative electrode significantly decreases during over-charge. As a result, there is a problem of a decrease in safety. The reason will be described below. In a battery in which the capacity of the negative electrode is relatively lower than the capacity of the positive electrode, when the battery is fully charged, the negative electrode is in a fully charged state. However, the positive electrode is in a state capable of being charged further. In the case where the battery in such a fully charged state is further charged, thereby reaching an overcharged state, the potential of the negative electrode becomes rapidly decreased. A significant decrease in the potential of the negative electrode causes a side reaction between the surface of the negative electrode and an electrolyte solution to be accelerated, and heat may be generated, accompanied with the side reaction. The heat increases the temperature of the interior of the nonaqueous electrolyte battery, which may result in a decrease in the safety of the nonaqueous electrolyte battery.